A study was conducted to determine rumen degradability (in sacco) of dry matter and in vitro gas production of the most important forage species grown in Iran, to compare forage species according to calculated degradation and in vitro gas production parameters, and to establish prediction equations for relative feed value (RFV) from gas production parameters. Thus, six forage species consisting of Lucerne, Eruca sativa, Crocus sativus, Cardaria draba, Setaria Spp., and Triticum aestivum forages were evaluated. Crude protein (CP) contents in the forages ranged from 139.60 to 246.30 g kg-1 DM. The neutral detergent fiber (NDF), acid detergent fiber (ADF) and organic matter (OM) were 273.30 to 596.60, 210.00 to 310.00, and 820.00 to 946.70 g kg-1 DM, respectively. The highest DM degradation and in vitro gas producton parameters were found for E. sativa. Also the relative feed value (RFV), relative forage quality (RFQ), dry mater intake and effective dry matter digestibility calculated for E. sativa was significantly (P < 0.05) higher than other forages. The variation of RFV explained by the gas production parameters ranged (R2) from 0.023 to 0.846. The gas production at 6, 24 and 48 h incubation times explained 0.836, 0.800 and 0.805 of variation of RFV, respectively. There was a negative correlation between in vitro gas production in different time incubation with NDF, ADF and a positive correlation between gas production parameters and CP content of forage species. The study shows that these forages relatively had a good nutritive value in comparison of Lucerne, and therefore, may serve as potential supplements for ruminants in Iran, and it seems that RFV index of six forage species such as used in this present study may be predicted from in vitro gas production parameters.

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